Electrolysis of aqueous salt solutions in liquid cathode cells



8. w. HIRSH ELECTROLYSIS OF AQUEOUS SALT SOLUTIONS IN LIQUID CATHODE CELLS Filed Sept. 9, 1946 INVENTORS BENJAMIN W. HIRSH CHARLES CARTER ATTORNEYS Patented Feb. 20, .1951

UNITE-D STATES .zELECTROLYSIS -OF PATENT OFFI AQUEOUS SALT SOLU- .TIONSIN LIQUID CATHODE CELLS Application, September 9, 1946;Serial No In Great Britain August-27,1941

' Sectionl; BublicLaw .69 0, Augustt8,:-1946 v.Baient; expires Augustl27,.19 61 "2 Claims. .1 ;-This invention, relates to ;an improved process for :electrolysis, and :more particularly to an improved process for the: electrolysis ofaqueous solutions of alkali metal chlorides .using a mercury .cathode. .TIhe manufacture of chlorine andsolutions. of caustic soda of high purity by electrolyzingaque- -ous solutions ofsodium chloride-using a.,-mer- -c ury cathode, andsubsequently, decomposing; the qsodium amalgam which is formed, is a well known industrial process. :Many .forms of apparatus have been proposed for carrying out such processes and, in fact,-it--appears that substantially ,all advances which have been vmade in this art for a considerable time pasthave involvedrefine- :ments in the construction of'the cell'structures used in these processes. "Notwithstanding these various 'refinementsthe electrolysis of aqueous saline solutions using liquid electrode cells "has heretofore involved certain undesirable features, e. g relatively large voltage and currentrequirements and, a1so, 1ow efiiciency of operation.

'A principal object of this inventionis the'pro- .vision ofa nevvand improved'process for the electrolysis of aqueous saline solutions inflowing, liquid electrode cells. Another object is the provision of electrolyticrprocesses Which require a relatively low operating-"voltage. -A still'further object is theprovision of such processes-Which are capable of close control upon the reactions;

taking place in the'electrolytic cell. Still further objects-and the entire scope-oiwapplicability'of this invention will become apparent from the detailed description givenhereinafter. 'These objects are accomplished according to the process of the present invention by operating the electrolysis of aqueous solutions in liquid elecf'trode :cells, so that the-'electrolyte-is caused-to solid anode and the flowing liquid cathode at'a high velocity relative to the anode. "The success of the present -invent on depends to a la marked'reduction in the operating voltage incells of the subject type is obtained by: increasing the velocity pf the electrolyte flow in the inter 'electrode gap above a req'u-ired highvelocity. *While the vabsolute "reason for the unusual results to'btained by theuse --of this "discovery-are-not I certain; it appears:;that,-lamon other things;- the high velocityzelectrolyte fiow'ai'dsin the-removal of gasbubbles from the surfaceof the solid anode causing a diminution nof the operating voltage requirements: .ofcthezoell. :Thus,=in operating. cells of the subject type with electrolytesspeeds which *flow through the inter-electrode space between a rge extent uponthe discovery that a' very J2 have been considerednecessary heretofore,- i. e., normally 1.,below .2 ,cm. per second and ,usually "between 10.25 and 1 cm. perisecond -stationary bulbs of.chlorinegas collect. on the under-surface .5 of the-anode and. increase the electrical resistance 'of the apparatus requiring highernoperatingvoltage than, should theoreticallybe needed. It has ,been found that these. bulbs .ofgas persistleven .with electrolyte velocities. substantially. inexcess l of the customary values, as indicated-above but it has now further been found that,,as.the.interelectrode velocity .of the solution is increased above a valueof. the. order of. .7..5. cm.. per. second, thata unique diminution ,.of theoperatin voltage l5 of the cell results apparentlyedue.to the lackcoi' .gstationary gas bulbs upon the. anode ,surface. Precise measurements iofithe velocity of theelec- "trolyte are not easy .to makeiinpractice,..but.rfor any given cell the .conditionsrof the invention may be determine'dgby Varying.thev electrolyte. flowin v the inter-electrode gap and noting. the correspondingcell voltage. lFlactorsrother-than gas bubble prevention also are probably involvedtin theoperation of. the uniquefeatures ofthisinven- .25 tion but, .iwhateverimay .be the. explanation. it .is a factthathasiaicertain brine velocity isreached, itiis found. that ,a.diminution .in thetcell voltage occurs, and it can be taken that, -.when.this..has occurred, asuitableelectrolyte velocity has-vbeen reached. In accordance With.. the present ;-inven- (tion it is possible to operate-the electrolysis consistently at a voltage of from. 3.5 to 3;? .and cathode currentndensity. in excess .oflZOamperes persquare foot, e, -,.15.0-.-.amperes,per squarefoot as compared with theistandard operationof from "4 to 5 volts under such condition of current density. "Further increase in the velocity ofthesolution produces, some further diminution in cell voltage I due to'the. diminution reached vin the ratioof. the ;volume of gas; to the volume of solution inQthe electrolyte. "With. still further increasesinj the velocity, the further gain in energy consumption duet-o this cause "would tend to :be counter- "balanced by the'lncreased cost of circulating ---electrolyte.

' The general mode of operation of-electrolysis of saline solutions to produce caustic soda and chlorine of high purity according to the procedure of this invention may be'briefiy stated as follows: "The apparatus used in the process preferably consistsof a trough-shaped'vessel with a smooth plane bottom. A-vessel is used which-is longin Mcomparison with its width" and is arrange f l ghtangle to the horizontal sothat the liquid ;,9 also lined with insulating material 5.

high velocity of the electrolyte flow which is esi sential to the process, the electrolyte is either introduced into the cells under super-atmospheric pressure or the outlet of the cell is maintained under sub-atmospheric pressure. In order to accomplish this, the cell is provided with a suitable cover and means for sealing the cell from the atmosphere. Such cells are, for example, described in copending application, Seri l No. 695,622, for Electrolysis, filed by John Gl ss and Charles Carter, on September 9, 1946; Serial No. 695,623 for Elec rolytic Cells, filed by Charles Carter and Arthur W. Ravenscro t. on September 9, 1946, and in U. S. Patent No. 2,503.337. The liquid cathode employed in the o eration of th cell is preferably mercury, but it should be realized that, as the electrolysis proceeds. the pure meal becomes amalgamated with the sodium produced in the process, so that the amalgam formed continuously increases in strength. Therefore, the word "mercury as used in this specification and the appended claims, should be understood to cover the resulting amalgam as well as the metal, unless the context requires the narrower meaning. In use, the mercury is caused to flow throu h the cell at a controlled rate wi h the saline electrolyte solution flowing through the cell as indicated herein in the gap formed between the flowing mercury cathode and the station r solid graphite anodes. The chlorine evolved in the inter-electrode space during the process is removed at a suitably spaced interval rom the inter-electrode space by means of suitably arranged exhaust pipes.

In order to give a bett r understanding of the nature of the apparat s which can be used to carry out the new methods of this invention, one

usable form thereof is illustrated in the attached dra in in which:

The figure is a vertical sectional view of a form of cell having skirts de ending from the cell cover and sealed to a fla base by means of a resilient sealing ring or member.

Referring in det il to the drawing, the flat base portion 1 carries the flowing mercury cathode 2 and the aqueous electrolyte 3 above the mercury cathode.

The cell has a cover 4 of steel, lined with insulating material 5, resistant to the products of electrolysis. Cover 4 carries a number of solid carbon anode blocks 6 (one of which is sho n) each depending from carbon rods i which p ss through openings in the cover 4 and through which current is supplied to the anode from a current source (not shown). In this form of cell, the anode rods 1 are sealed by a sealing composition 8, but other suitable methods for making a gastight joint-may be employed.

The cover 4 is provided with a depending skirt These skirts extend down below the level of the mercury cathode 2 and a liquid-tight joint is formed between the depending skirts and the base portion I by an endless band of sponge rubber l having an impermeable outer skin carried in the semi-circular peripheral groove ll formed in the base portion I.

On the sides of the cover 4 are horizontal lugs i2 which serve to support the cover 4 at any desired height upon the pillar-bolt l3 threaded into the stand 14. The collars l5 and locknuts 16 permit the height of the cover 4 relative to the base I to be properly adjusted.

The space running the full length of the cell between the anode cover 4 and the anode 6 is freely accessible to the electrolysis gases which are led out of the cell by one or more gas outlet tubes H which are sealed to the cell cover by sealing composition 8. Although there is free space above the anode, the actual flow of electrolyte is substantially restricted to the inter electrode gap by the process gas pressure above' the anode and by placing the electrolyte exit horizontally in line with the gap.

The drawing is diagrammatic and employed only to allow for better comprehension of the written description of the invention. Different forms of apparatus can be used in carrying out the new process, as has been indicated above, and further discussion and illustrations of usable apparatus can be found in the patent applications and patents listed above.

The operation of my invention may be more fully comprehended by reference to the following illustrative examples in which all parts are by weight, unless otherwise specified.

Example I Electrolysis of an aqueous solution of sodium chloride is accomplished in an electrolytic cell long in comparison with its width having a plane bottom surface consisting of a flat steel plate, inclined at a slope of 1 in 300 to the horizontal, adapted to carry a flowing mercury cathode. The cell has also a cover provided with a depending, rigid skirt adapted to fit loosely over the flat steel plate. A resilient packing, such as sponge or inflated rubber is positioned in a peripheral groove on the steel plate so as to make a liquid-tight seal between the skirt and the steel plate. Adjusting means are also provided whereby the cover can be maintained at any desired height with reference to the steel plate which carries the mercury. Graphite anode blocks are arranged to fit closely to the cover and the inside wall of the skirt and are provided with suitable slot or perforations surmounted by ofi-take pipes, each extending to a height above the cell greater than the pressure of brine generated below the off-take, and thus forming vents to permit the escape of the chlorine the graphite anodes and the steel surface carrying the mercury is about 6 mm.

An aqueous solution of sodium chloride of 25% concentration is supplied to the cell under a hydrostatic head corresponding to 0.036 lb. per square inch per foot length of the cell. The velocity of the brine along the cell relative to the anode surface, calculated on the assumption that the brine flow is restricted to. the interelectrode space, is 42 cm. per second. The true velocity is somewhat less than this figure owing to inaccuracies in the fitting of the anodes to the cover which lead to some bypassing of the brine flow. The cell operates at a voltage of 3.51 volts and a current of 12,000 amperes when the brine temperature is C.

Example 11 Using the apparatus of Example I an aqueous sodium chloride solution of 25% concentration is electrolysed with an identical graphite to steel gap with the electrolyte introduced into the cell under a hydrostatic head corresponding to 0.024 lbs. per square inch per foot length of cell. The linear velocity, calculated as above, is 21 cm. per second. Using a cathode current density of 149 amperes per sq. ft. the gas volume to solution volume in the fluid leaving the interelectrode space is approximately 1 to 5.8 and the cell voltage is 3.63 volts.

The velocity which gives satisfactory values for the gas/solution ratio will vary with such factors as the current supplied to the cell, the interelectrode gap and the distance between successive points at which chlorine can escape from under the anode; in practice, however, it is found that the velocity chosen with respect to gas/solution ratio is always greater than that which suffices to remove the stationary bubbles of chlorine from the lower face of the anode.

The velocity in the inter-electrode space may be attained by increasing the slope of the cell bottom sufiiciently; however, the necessary high slope will entail loss of current efiiciency which may ofiset the gain due to reduced cell voltage. Preferably, therefore, a cell is employed which is capable of operating under conditions involving the maintenance of a pressure within it different from atmospheric, and the increased velocity in the inter-electrode space is secured by increasing the hydrostatic head under which the solution to be electrolyzed is delivered to the cell. In using a mercury cell of the type hitherto employed in which there is a gas space above the anodes, difficulties arise due to the tendency of the electrolyte to flow above the anodes and unduly high pressures would be required to achieve the desired velocity in the inter-electrode gap. Advantageously, therefore, the process of the invention is carried out in such a manner that the electrolyte is caused to flow through the cell solely through the inter-electrode gap, and the evolved chlorine gas is drawn off at intervals from the inter-electrode gap. A suitable form of cell for the process is one in which the anode fills substantially the whole of the cell above the inter-electrode gap, vents for the chlorine being provided each extending to a height greater than the head of electrolyte developed beneath it before joining a common header. In such a cell, there is a close correlation between the pressure difference between the ends of the cell and the velocity of the solution being electrolyzed in the gap. This pressure difierence depends to a large extent on the conditions and on the size of the cell. It is, therefore, not practicable to specify the pressure difierence to be used until the conditions are known. However, when it is apparent that suitable velocity conditions in a particular installation are established, then it is found more convenient to use the pressure drop, which can be directly observed, to verify that suitable operating conditions are maintained in the cell, rather than to rely on a determination of the velocity in the narrow inter-electrode gap. The latter must necessarily be made indirectly from measurements of the volume of solution delivered to the cell in a given time, and from the geometrical configuration of the cell.

In using the apparatus for carrying out the process of the present invention, an alkali metal chloride solution is supplied tothe cell under a comparatively large hydrostatic head so that the velocity of the solution between the electrodes is above the required high velocity as hereinbefore defined. Alternatively, the electrolyte may be supplied at atmospheric pressure and the increased velocity achieved by applying suction to the exit by which the spent solution leaves the cell.

As many and varied modifications of the subject matter of this invention will become apparent to those skilled in the art from the detailed description given herein, it should be understood that this invention is to be limited only in accordance with the appended claims.

I claim:

1. The process for the electrolysis of aqueous solutions of alkali metal chlorides between a closely spaced solid graphite anode and a flowing mercury cathode which comprises restricting the flow of the electrolyte substantially to the interelectrode space and causing the electrolyte flow to exceed a velocity, relative to the anode, of 7.5 cm. per second by maintaining a fluid pressure differential between the inlet and outlet of the inter-electrode gap sufficient to cause said flow.

2. The process according to claim 1 in which said electrolyte flow above said critical velocity is obtained by restricting the fiow of electrolyte substantially to the inter-electrode space and creating a substantially higher pressure upon the inlet end than upon the outlet end of said space.

BENJAMIN WOOLF' HIRSH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 903,951 Billiter Nov. 17, 1908 1,961,160 Moulton June 5, 1934 2,224,814 Gilbert Dec. 10, 1940 2,234,967 Gilbert Mar. 18, 1941 2,232,128 Muller Feb. 18, 1941 2,467,892 Horst Apr. 19, 1949 FOREIGN PATENTS Number Country Date 7,476 Great Britain of 1898 17,415 Great Britain of 1898 471,912 Great Britain of 1937 265,414 Germany of 1913 OTHER REFERENCES Principles and Applications of Electrochemistry, by W. A. Koehler, vol. II, 2nd Edition, 1944, pages 315-317, 355, 356.

The Principles of Applied Electrochemistry, by A. J. Allmand, 2nd Edition, revised by H. J. T. Ellingham, 1924, pages 398, 400. 

1. THE PROCESS FOR THE ELECTROLYSIS OF AQUEOUS SOLUTIONS OF ALKALI METAL, CHLORIDES BETWEEN A CLOSELY SPACED SOLID GRAPHITE ANODE AND A FLOWING MERCURY CATHODE WHICH COMPRISES RESTRICTING THE FLOW OF THE ELECTROLYTE SUBSTANTIALLY TO THE INTERELECTRODE SPACE AND CAUSING THE ELECTROLYTE FLOW TO EXCEED A VELOCITY, RELATIVE TO THE ANODE, OF 7.5 CM. PER SECOND BY MAINTAINING A FLUID PRESSURE DIFFERENTLY BETWEEN THE INLET AND OUTLET OF THE INTER-ELECTRODE GAP SUFFICIENT TO CAUSE SAID FLOW. 